1. Field of the Invention
The present invention relates to an image forming apparatus based on an electrophotographic system and having a blade cleaning device.
2. Description of the Prior Art
A number of methods and apparatuses have been proposed for obtaining a color image by means of an electrophotography. As disclosed in the Official Gazette on Japanese Patent Laid-Open No. 100770/1986, for instance, there is a method wherein latent images in the number corresponding to those of separating colors of an original image are formed and developed on a photosensitive drum (an image retainer) and transferred onto a transfer drum at every time of development and, after a multicolor image is thus formed on this drum, it is transferred onto a transfer material at one time, so as to obtain a color copy. An apparatus based on this method needs to be provided with the transfer drum being large enough to allow an image for one sheet to be transferred on the peripheral surface thereof, in addition to the photosensitive drum, and therefore the apparatus has inevitably a large-sized and complicated structure.
Besides, as disclosed in the Official Gazette on Japanese Patent Laid-Open No. 149972/1986, for instance, there is another method wherein the latent images in the number corresponding to those of the original copy are formed and developed on the photosensitive drum (the image retainer)and transferred onto a transfer material at every time of development, so as to obtain a multicolored color copy. In this method, it is difficult to superpose multicolored images precisely and consequently it is impossible to obtain a color copy of excellent quality.
Moreover, there is a method wherein the formation of the latent image and the development with color toner are repeated plural times corresponding to the number of separating colors of the original image on the photosensitive drum and, after the color toners are superposed on the photosensitive drum, the superposed toner is transferred at one time onto the transfer material, so as to obtain the color image. Basic processes of this multicolor image formation are disclosed in the Official Gazettes on Japanese Patent Laid-Open Nos. 75850/1985, 76766/1985, 95456/1985, 95458/1985 and 158475/1985, etc. In multicolor image forming apparatuses designed to obtain the color image by such super-position development as described above, color images of excellent quality wherein color shift is little can be obtained.
In the above-mentioned image forming apparatuses, a multicolor image is formed by the superposition development and a toner image formed on the photosensitive drum is transferred on transfer paper, while residual toner is cleaned up to prepare subsequent image formation.
As for a cleaning device, a variety of methods and devices have been proposed. As disclosed in the Official Gazette on Japanese Patent Laid-Open No. 147047/1979, for instance, there is a cleaning method of electrophotography wherein, in an electrophotographic copying apparatus wherein a remaining toner "residual toner" on the surface of a photosensitive member is removed by a cleaning blade, the cleaning blade comes into pressure contact with the photosensitive member at least before the photosensitive member is moved and the blade is detached from the surface of the photosensitive member after the member stops moving after the completion of a copying process. Besides, a blade cleaning device has been in wide use recently, wherein a cleaning edge e (a contact element) of the end part of a flat-plate-shaped cleaning blade 19A as shown in FIGS. 6(a) and 6(b) is pressed on a photosensitive drum 10 (an image retainer) so as to clean up residual toner, since the device is simple and since the toner scatters little in cleaning.
The blade cleaning device is sorted into a counter type wherein a support member 19C of the cleaning blade 19A (hereinafter called simply a blade) is positioned on the downstream side in relation to the rotation of the photosensitive drum 10 as shown in FIG. 6 (a), and a trail type wherein the support member 19C of the blade 19A is positioned on the upstream side as shown in FIG. 6 (b). The counter type is a little more excellent in a cleaning efficiency than the trail type, a load on the photosensitive drum at a contact point can be reduced, and a change in the speed of rotation of the photosensitive drum is small. Therefore, said type causes color shift little and so it is often used.
In FIG. 6 (a), mark .theta. denotes a contact angle which is an angle formed by a tangent of the photosensitive drum 10 at a contact part of the blade 19A and by the blade 19A, and t denotes the thickness of the blade 19A, while l denotes a free length from the support member 19C of the blade 19A to the fore end thereof.
The above-mentioned blade 19A is always left to be in pressure contact with the photosensitive drum 10 in some cases. However, in the apparatus wherein the above-mentioned superposition development is conducted, for instance, it is necessary to make it possible to bring the blade into pressure contact and to release the same therefrom in such a manner that the blade 19A is detached from the photosensitive drum 10 during the formation of a plurality of toner images, brought into the pressure contact for cleaning after the completion of a transfer process and then detached again when the cleaning is ended.
As for an image exposure device of the above-mentioned image forming apparatus, the device using a semiconductor laser has increased recently. In this device, laser beams modulated by image informations obtained by reading an image on an original are applied on the photosensitive drum 10, an electrostatic latent image is formed thereby, and this electrostatic latent image is developed by a developing means.
In the image forming apparatus using the laser beams, an image forming method of a reversal developing system is used generally wherein the laser beams are applied on the photosensitive drum 10 charged uniformly and toner is made to stick on a place wherefrom charge is eliminated by the application of the beams. In this method, the time of employment of a laser light source is shortened advantageously and, in addition, the image quality of a copy thus obtained is improved (the effect is remarkable especially when the original is a document). Further, a part outside an image area wherefrom the charge is not eleminated is left charged in the reversal development.
The above-mentioned blade 19A sometimes causes disadvantage that its contact part vibrates irregularly and collides with the photosensitive drum 10 (this phenomenon is called bounding) when it conducts a cleaning operation in pressure contact with the photosensitive drum 10, causing imperfection in cleaning or a damage of the blade 19A itself (a fine split of the contact part) or the occurrence of a flaw in the surface of the photosensitive drum 10. In the apparatus wherein the above-mentioned superposition development is conducted, in particular, the pressure contact and release of the blade 19A are conducted incessantly and the above-mentioned bounding tends to occur on the occasion of the pressure contact and release. Besides, this problem tends to occur also in the case when the reversal development is conducted with the blade 19A in the counter type. The bounding occurs also in an ordinary image forming apparatus, and especially when a large number of copies are prepared continuously in the condition that the size of a transfer material is small in comparison with the width of the photosensitive drum 10, this bounding occurs at a high rate.